Glacier geology still cleaning Iowa groundwater

May 01, 2005

The geology left by the last glacier to advance into Iowa is helping to clean nitrate from Iowa groundwater and producing methane gas, according to research by William Simpkins, an Iowa State professor of geological and atmospheric sciences.

That last ice advance, called the Des Moines Lobe, entered north-central Iowa about 14,000 years ago and flattened wet, boggy forests of spruce and tamarack that were growing on the loess-covered landscape at that time. When the Des Moines Lobe melted about 12,000 years ago, it left that flattened loess landscape buried under the glacial till seen across central and north-central Iowa today.

Simpkins said the buried logs and peat in the till and loess are a source of organic carbon. When they decompose and dissolve into the groundwater, they become a food source for bacteria. To consume it, bacteria have to oxidize the organic carbon. They first look for oxygen to do that. But in a low-oxygen environment such as the groundwater under north-central Iowa, they next turn to nitrate, then to iron and then to other elements and compounds. The result is that nitrate in the groundwater is reduced to gas. And when the bacteria eventually turn to carbon dioxide to oxidize the organic carbon, they create methane gas as a byproduct. Methane gas is common in groundwater anywhere from 15 to 300 feet deep underneath the area covered by the Des Moines Lobe.

And where there's methane gas in the groundwater, Simpkins said there shouldn't be nitrate.

Simpkins will present his findings at a meeting of the North-Central Section of the Geological Society of America May 19-20 in Minneapolis.

Iowa farmers use various forms of nitrogen as a corn fertilizer. Some of that nitrogen leaks into Iowa's shallow groundwater and often into drinking water as nitrate. The Statewide Rural Well Study in 1988 and 1989 showed that 18.3 percent of groundwater samples from 686 private wells had nitrate concentrations greater than the Environmental Protection Agency limit of 10 milligrams per liter as nitrogen. More recently, 133 samples taken from 15 incorporated communities in the spring and summer of 2003 as part of the Iowa Community Private Well Study showed 23.3 percent of those wells had nitrate concentrations above the EPA limit. And Iowa streams typically have two to 10 times the levels of nitrogen considered appropriate for Midwest steams, according to a report by the Iowa Department of Natural Resources.

Water quality in aquifers beneath the Des Moines Lobe would be a lot worse if bacteria weren't using the nitrate to oxidize organic carbon, Simpkins said.

Based on estimates of the organic carbon in the groundwater by Simpkins and co-author Timothy Parkin, a research microbiologist with the National Soil Tilth Laboratory based at Iowa State, the nitrogen-scrubbing system created by the Des Moines Lobe can keep cleaning groundwater for about another 565 years before the organic carbon is exhausted. But Simpkins said that is an optimistic estimate based on very limited data. The nitrate-cleaning capacity may be significantly less in some areas.

This article originally appeared in the 05/01/2005 issue of Environmental Protection.